Found this thread while experiencing the same issue on 2022.3.93. Posting my findings in the hope it'll help someone else.
The issue for me was caused by some rogue HTML at the beginning of a snippet that appeared in the topic. This wasn't HTML that I pasted from elsewhere. I created every topic (and every snippet) from scratch for the entire document using a 2022 version of RH. It was RoboHelp going rogue!
There was a bug in an early version of RH2019 New UI where I managed to drag and drop the entire RH interface into my topic. But Adobe did fix that one rapidly. 😄 So if you created/modified those imported topics in one of the early versions, that might be how the code got in.
I've no idea how that code got in there as it is for the line that should only appear when you hover, it should not add anything to the code. One of life's mysteries but now resolved. Was it just the one topic?
I had a total of 4 of pages,each page was a separate page of it's own. What I had done was, I copied a page to create a new page and just modified the page.
Anyhow, I'm glad I didn't have to create 4 new pages.
I'm always modifying the Source Code, so I feel comfortable enough to troubleshoot for issues like this.
I thought maybe there would have been something that I need to do to either hide that dotted line or click on an option to remove the line.
The only dotted line I have seen like that in the new UI versions is in the authoring panel. You cannot stop it appearing there. It is not something the end user will see.
There are some scenarios where pressing enter above it did not create a new paragraph, then you had to click the icon on the right of the line to create a fresh line.